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Root Cause Investigation of Sub-Synchronous Vibration in a …€¦ · Root Cause Investigation of...
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Root Cause Investigation of Sub-Synchronous Vibration in a Multi-stage Centrifugal Compressor
Jun Inai, Project & Development Engineer
Kawasaki Heavy Industries, Japan
Outline
• Introduction
• Findings
• Root Cause Analysis
• Modification
• Validation of modification
• Site confirmation test
• Conclusion
• Lesson learnt
Introduction
During the commercial operation of an eight-stage back to back GT driven centrifugal compressor locates at the end-user’s off-shore platform, high level shaft vibration alarm under specific operating conditions was reported from end user.
According to the site evaluation test with dynamic measurements, sub-synchronous vibration (SSV) was observed under higher load conditions of high pressure compressor for every operating speed.
This case study features the root cause analysis of SSV problem using large scale unsteady CFD and the final result of site confirmation test after improvement.
Findings (1) Site Evaluation Test
GAS
COOLER KNOCK OUT
DRUM
Casing drain
GAS
COOLER
PT TT
FT TT
PT
DPT
KNOCK OUT
DRUM
TT PT
FT TT
PT DPT
LP COMPRESSOR(LPC) HP COMPRESSOR(HPC)
XE XE
ANTI SURGE VALVE ANTI SURGE VALVE
Site evaluation test was conducted to understand the circumstances. Dynamic measurements of rotor vibration and discharge pressure at HPC casing drain and downstream piping were implemented.
GT
DPT XE : Dynamic measurement points
Pressure transducer
Vibration transducer
Findings (2) Operable Range and Shaft Vibration
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
0 1000 2000 3000 4000 5000 6000 7000 8000
Suction Volume Flow - m3/hr(corr)
Pre
ssu
re R
atio
Pre
ssure
ratio
SSV not present
105% speed
100% speed
80% speed
90% speed
Suction volume flow
97.9% speed
Shaft vibration alarm detected !!
SSV onset line
Operable range was restricted by high level shaft vibration.
SSV onset points correspond to increase of shaft vibration.
HPC operating map
Findings (3) Sub Synchronous Vibration
SSV around 20~30 Hz were dominantly present. They were approximately 1/6~1/7 times the machine rotational speed. Same frequency of discharge pressure fluctuation were also detected at casing drain and down stream piping. Is this a typical vaneless diffuser rotating stall ? At first we suspected it as the most possible root cause.
80% speed 90% speed 100% speed
Observed radial shaft vibration (Y-NDE)
16μm
1N =146Hz
28Hz
8μm
18Hz 20Hz 1N
=130Hz
1N =164Hz
Root Cause Analysis (1) Rotating stall check at vaneless diffuser inlet
Rotating stall at diffuser inlet is checked at the design phase
based on Senoo criteria. And it was re-confirmed that sufficient acceptable
margin were secured.
Flow angle at diffuser inlet at design condition
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
34.0
36.0
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
b2/r2
Flo
w a
ngle
at diffu
ser
inle
t (d
eg)
Flow angle of each stage of HELANG
Critical flow angle, rotating stall to occur
Stable
Unstable
HPC Senoo criteria
Flo
w a
ngle
at diffu
ser
inle
t [d
eg]
LPC
It indicates the root cause is not a rotating stall at diffuser inlet.
Impeller exit blade height / Impeller diameter b2/r2
Root Cause Analysis (2) CFD Analysis of 8th stage
IGV
Impeller (φd=0.0118)
Parallel wall vaneless diffuser
Diffuser outlet with spacer vane
Eye seal
Center seal
Discharge Volute
Shunt holes
Deflector
• Number of vane - Impeller = 22 w/t splitter - IGV = 16 - Spacer vane = 8
• Number of shunt holes = 3 • Rotating speed =9514 rpm • Calculation time = 1day/rotation
Birdview from upstream
from 7R
to 4R
Center seal leakage
Rabbet fit
Large scale unsteady CFD analysis was carried out for the 8th stage.
Root Cause Analysis (4) Static pressure fluctuation at spacer vane inlet
HzmT
f
HzT
f
T
Tm
r 2310145.441
11
2310145.44
11
11.110145.44
10248.10
75
360360
3-
3-0
3-
-3
2 rev.
000000ddeegg
006600ddeegg
118800ddeegg
336600ddeegg
112200ddeegg
224400ddeegg
330000ddeegg
ΔT=10.248ms ΔT=10.248ms
ΔΔθθ==7755°°
1N
ΔT=10.248ms
T=44.145ms
TIME
LO
CA
TIO
N
HzmT
f
HzT
f
T
Tm
r 20103065.68
11
159106.3065
11
88.7103065.6
10248.10
75
360360
3-
3-0
3-
-3
HzmT
f
HzT
f
T
Tm
r 2110766.153
11
6310766.51
11
31.310766.15
10248.10
75
360360
3-
3-0
3-
-3
Time & space distribution of static pressure at spacer vane inlet
Station VD5
000deg
090deg 270deg
180deg
Rotation
Cir
cum
fere
nti
al p
osi
tio
n
20Hz 21Hz 23Hz
Close frequency as observed at site test could be simulated.
Root Cause Analysis (5) Static pressure fluctuation at other stationary region
Static pressure time & space distribution & FFT spectra
Strong pressure fluctuation at spacer vane inlet affects to the other stationary region.
N=17.5TH REVOLUTION N=18.0TH REVOLUTION N=18.5TH REVOLUTION
N=19.0TH REVOLUTION N=19.5TH REVOLUTION N=20.0TH REVOLUTION
Root Cause Analysis (6) Unsteady pressure distribution across the stationary region
Static pressure distribution
Periodic pressure fluctuation was shown across the whole stationary region.
Root Cause Analysis Summary
A large scale unsteady CFD analysis achieved to simulate the sub-synchronous phenomena as close frequency as measured SSV at site test and indicates strong flow fluctuation due to large flow separation at the diffuser outlet with spacer vane at the final stage.
It was considered that the root cause is complete stall induced from diffuser outlet due to excess flow passage expansion between diffuser outlet and discharge volute at the final stage.
Therefore, configuration of ‘diffuser outlet with spacer vane’ shall be re-designed.
Modification Improved diaphragm of 8th stage
Cross-sectional configuration of diffuser outlet was changed from expanded shape to parallel wall shape
Spacer vane shape was also changed from cusped to elliptical blunt.
Spacer vane
Pre-modified Modified
Validation of modification (1) Static pressure fluctuation at other stationary region
Static pressure time & space distribution & FFT spectra
Confirmed no presence of noticeable time & space distribution of static pressure at spacer vane inlet and other stationary region.
N=17.5TH REVOLUTION N=18.0TH REVOLUTION N=18.5TH REVOLUTION
N=19.0TH REVOLUTION N=19.5TH REVOLUTION N=20.0TH REVOLUTION
Validation of modification (2) Flow stability across the stationary region
Static pressure distribution Pressure fluctuation at the stationary region completely disappear.
Validation of modification (3) Rotor excitation force
Pre-modified Modified
View from upstream View from upstream
Rotor excitation force at the stage 8th occurs in the direction of discharge nozzle.
Excitation force time averaged/dynamic have both decreased in association with modification.
Dynamic Dynamic
Site confirmation test (1) Operable range & Shaft vibration
Surge line
Confirmed wide operable range is secured as estimated
Overall vibration is less than 25μm for whole operable range
Pre
ssu
re r
atio
105% speed
100% speed
80% speed
90% speed
Suction volume flow
97.9% speed
93.4% speed Former SSV onset line
Surge control line
Site performance test (2) SSV presence
Pre-modified Modified
16μm
1N =164Hz
28Hz
20μm
SSV Negligible small (<1μm)
Confirmed no dominant SSV presence for all operable range.
5μm
1N =160Hz
Conclusion
With regard to the natural gas export compressor on the off-shore platform which was restricted its operable range due to SSV as 1/7 times the machine rotational speed, a large scale unsteady CFD analysis was carried out in order to investigate the root cause.
The CFD analysis achieved to simulate those sub-synchronous phenomena. And it was found that the root cause was a typical stall at diffuser outlet due to excess flow passage expansion between diffuser outlet and discharge volute at the final stage.
Modified stationary flow passage was designed and validated its effectiveness by CFD analysis in the same manner as root cause analysis.
Modified diaphragm was already installed to the site machine. The followings were confirmed through the site evaluation test. * No presence of dominant SSV for whole operable range * Operable range is secured as estimated
Lesson & Learnt
Even the stalls in such a stationary flow passage region apart from the rotor can be the excitation force of shaft vibration especially under high pressure condition.
Sufficient consideration and care with a broad view shall be taken during the engineering phase.